Wireless communication systems commonly include information-carrying modulated carrier signals that are wirelessly transmitted from a transmission source (for example, a base transceiver station) to one or more receivers (for example, subscriber units) within an area or region.
A form of wireless communication includes multiple transmit antennae and/or multiple receiver antennae. Multiple antennae communication systems can support communication diversity and spatial multiplexing.
A Wireless Channel
FIG. 1 shows modulated carrier signals traveling from a transmitter 110 to a receiver 120 following many different (multiple) transmission paths.
Multipath can include a composition of a primary signal plus duplicate or echoed images caused by reflections of signals off objects between the transmitter and receiver. The receiver may receive the primary signal sent by the transmitter, but also receives secondary signals that are reflected off objects located in the signal path. The reflected signals arrive at the receiver later than the primary signal. Due to this misalignment, the multipath signals can cause intersymbol interference or distortion of the received signal.
The actual received signal can include a combination of a primary and several reflected signals. Because the distance traveled by the original signal is shorter than the reflected signals, the signals are received at different times. The time difference between the first received and the last received signal is called the delay spread and can be as great as several micro-seconds.
The multiple paths traveled by the modulated carrier signal typically results in fading of the modulated carrier signal. The fading can be time selective and cause the amplitude and phase of the modulated signal to vary with time. The fading can also be frequency selective and cause the frequency response of the channel over the signal bandwidth to vary with frequency.
Spatial Multiplexing
Spatial multiplexing is a transmission technology that exploits multiple antennae at both the base transceiver station and at the subscriber units to increase the bit rate in a wireless radio link with no additional power or bandwidth consumption. Under certain conditions, spatial multiplexing offers a linear increase in spectrum efficiency with the number of antennae. For example, if three antennae are used at the transmitter (base transceiver station) and the receiver (subscriber unit), the stream of possibly coded information symbols is split into three independent substreams. These substreams occupy the same channel of a multiple access protocol. Possible same channel multiple access protocols include a same time slot in a time-division multiple access protocol, a same frequency slot in frequency-division multiple access protocol, a same code sequence in code-division multiple access protocol or a same spatial target location in space-division multiple access protocol. The substreams are applied separately to the transmit antennae and transmitted through a radio channel. Due to the presence of various scattering objects in the environment, each signal experiences multipath propagation.
The composite signals resulting from the transmission are finally captured by an array of receiving antennae with random phase and amplitudes. At the receiver array, a spatial signature of each of the received signals is estimated. Based on the spatial signatures, a signal processing technique is applied to separate the signals, recovering the original substreams.
FIG. 2 shows three transmitter antenna arrays 210, 220, 230 that transmit data symbols to a receiver antenna array 240. Each transmitter antenna array and each receiver antenna array include spatially separate antennae. A receiver connected to the receiver antenna array 240 separates the received signals.
Communication Diversity
Antenna diversity is a technique used in multiple antenna-based communication system to reduce the effects of multi-path fading. Antenna diversity can be obtained by providing a transmitter and/or a receiver with two or more antennae. Each transmit and receive antenna pair include a transmission channel. The transmission channels fade in a statistically independent manner. Therefore, when one transmission channel is fading due to the destructive effects of multi-path interference, another of the transmission channels is unlikely to be suffering from fading simultaneously. By virtue of the redundancy provided by these independent transmission channels, a receiver can often reduce the detrimental effects of fading.
Multiple channel receivers are generally associated with spatial multiplexing, receiver diversity or interference cancelling. Each of the channel receivers generally includes weighting that is used to estimate a received data streams from the received information signals. Typically, the received information signal includes noise and interference that limits the accuracy of the estimated received data streams. The noise typically consists of a noise floor due to thermal effects and background noise. The interference is generally due to other transmitters transmitting within a common frequency band. The interfering transmitters can belong to the same communication system and interfere due to cellular frequency reuse, or the transmitters can belong to other communication systems that are located in nearby regions.
It is desirable to have a method and system for receiving multiple information signals at a common receiver. It is desirable that the receiver be adaptable to optimally receiving information signals that are dominated by interference or noise. The method and system should be easily implemented with existing multiple channel weighted receivers.